Richard,
I think you will struggle to get a pedal to tighten up an amp which is not inherently tight in itself.
The whole Jcm900 and 2000 series are not considered to be TIGHT high gain amps, but to be a little bit loose, which is of course good in its own way, as it flatters a less than exact technique. Boosting the front end with a pedal will give you more gain, but I am sure that is not what you are looking for.
You need an amp design that inherently gives a more precise and accurate note attack. It also needs good damping and a low(ish) noise floor. I don't think you'll get this from a 900, unless you mod it until it is basically a different amp. THese things are attained through various different design parameters in the amp.
Sorry, but I really don't think Mesa's or the 5150 (6505) is what you are looking for either. Either of these can give you a wall of brewtal distortion, but not the accurate, tight sound that I think you are looking for.
I stilll confess to not being a high gain expert, but I do know that many of the Diezel, Engl and VHT designs would fulfil what you are looking for.